In surgeries requiring a total hip joint replacement, both the acetabulum and the upper portion of the femur must be replaced, and appropriate materials must be selected as the replacement components. During the 1970's, the femoral prosthetic component was typically made of a metal such as stainless steel, alloys of Cr--Co--Mo and titanium, while the mating acetabular cup was typically made of ultra high molecular weight polyethylene (UHMWPE). However, it soon became apparent that the metal-UHMWPE coupling produced significant amounts of polyethylene wear debris in vivo. This debris has been heavily implicated in the osteolytic destruction of periarticular tissues and the subsequent loosening of the hip joint prosthesis. Consequently, the medical community began to consider replacement materials for the metal heads.
Because of the wear and debris problem, standardized testing methods were also developed to help compare the wear rates of candidate materials for hip joint prosthesis components. Determining the wear-related suitability of components for use in a hip joint prosthesis typically involves performing a standard pin-on-disc wear test, such as ASTM F 732 82, and characterizing its results by a normalized wear factor, k. The wear factor, k, is defined as the wear volume V (in mm.sup.3) of a material, divided by the product of the load P (in N) and the sliding distance X (in m). According to Saikko, Wear 166 (1993) 169-178, a wear factor of 10.sup.-9 mm.sup.3 /Nm is "extremely low" while a wear factor of 10.sup.-7 mm.sup.3 /Nm is "considerable".
From these wear tests, alumina was identified as a candidate replacement material. The reported wear factors k of the UWPE.backslash.alumina couple of about 10.sup.-7 mm.sup.3 /Nm to 10.sup.-9 mm.sup.3 /Nm were found to be superior to the metal.backslash.HMWPE couple. Accordingly, modular femur components comprising an alumina head taper fit to a metal stem were developed, and these components were coupled with both UHMWPE and alumina cups. Although the alumina-UHMWPE and alumina-alumina couplings produce less wear debris, the low strength of alumina (only about 600 MPa) has hindered its widespread acceptance. Because of the unreliability associated with the low strength of alumina, other ceramic materials have been considered.
Over the past five years, artificial hip joint prostheses having zirconia heads have gained acceptance in the medical community. As the strength of partially stabilized zirconia is typically between about 900-1300 MPa and its toughness is at least about 5 MPa m.sup.1/2, it is more reliable than alumina. In fact, the superior mechanical properties of zirconia has even enabled its use in small 22 mm heads. In all hip joint prostheses using zirconia heads, UHWPE cups have been used. See, for example, U.S. Pat. No. 5,181,929, assigned to Ceramiques Techniques Desmarquest; Willmann (of Cerasiv), Biomedizinische Technik 39(4) (1994) 73-78; Saikko, Wear 166 (1993) 169-178; Derbyshire, Med. Eng. Phys. 16 (1994) 229-236; Tateishi Mat. Sci. Eng. C1 (1994) 121-125; Japanese Patent Publication (Kokoku) 5-75423; Streicher (of Sulzer) "Bioceramics, Vol. 4", (1991) 9-16; and Schwartz (of Richards), "36th Ann. Mtg. ORS" 1990 483. Since the reported production of UHWPE debris has been extremely variable in this case (a wear factor k of 10.sup.-7 mm.sup.3 /Nm according to Streicher and Derbyshire to 10.sup.-9 mm.sup.3 /Nm according to Saikko), there remains a continuing need to identify materials for use in acetabulum cups which, when coupled with zirconia heads, produce even smaller amounts of debris than a UHMWPE cup.
Researchers studying the wear of zirconia-ceramic couplings have reported extremely high wear. For example, the results reported by Ludema, in "Advanced Ceramics for Structural and tribological Applications" (1995) 37-45, yield an alumina plate-zirconia pin total wear factor of about 10.sup.-5 mm.sup.3 /Nm, and a zirconia plate-alumina pin total wear factor of over 10.sup.-5 mm.sup.3 /Nm. The results reported by Medevielle et al., in J. Eur. Cer. Soc. 15 (12) (1995) 1193-1200, yield an alumina plate-zirconia pin total wear factor of about 10.sup.-6 mm.sup.3 /Nm (assuming a test period of 30 minutes). Tucci, in Wear, 172, (1994) 111-119, reports yttria tetragonal polycrystal ("YTZP") zirconia plate-alumina pin wear factors ranging from 10.sup.-5 mm.sup.3 /Nm to 10.sup.-3 mm.sup.3 /Nm. Lastly, the results of Sudanese, in Alumina vs Zirconium Oxide: A Comparative Wear Test, in "Bioceramics 1989", pp. 237-240, Oonishi H, Aoki M, Sawai L (eds), yield a wear factor for a zirconia ring-zirconia disc coupling which was over 5000 times worse than an alumina-alumina coupling.
Because of the high wear factors associated with zirconia-ceramic couplings, the hip joint prosthesis field has considered and specifically rejected a zirconia head-ceramic cup coupling. For example, in Clarke, Clin. Orthop. 282 (1992) 19-30, the author points to the relative inferiority of zirconia in wear tests and warns that "there may be a cause for concern given the potential to mix and match alumina ceramic cups with zirconia balls". In Willmann, supra, Table 3 specifically discourages using a zirconia head and an alumina cup and bases that conclusion upon clinical and technical studies. Sudanese, supra, concludes that ". . . zirconium oxide cannot be used for ceramic-ceramic coupling prosthesis, due to its low wear resistance. It may be the bioceramic material of choice to make ceramic heads in hip prosthesis with polyethilene! sockets." Although Kyocera JP Patent Publication 4303443 discloses an alumina head coupled with a cup having either an alumina or zirconia mating surface, its reference to a zirconia cup is merely prophetic and it does not disclose a zirconia head with a cup having an alumina mating surface. Since the wear mechanisms for the head and cup are different (see e.g., Medevielle's discussion and Ludema's results when the couples are switched), Kyocera's silence with respect to the zirconia head-alumina cup combination is notable.
Accordingly, there exists a continuing need for an acetabulum cup for use with a zirconia head which produces a wear factor of less than 10.sup.-7 mm.sup.3 /Nm.